1. Field of the Invention
The present invention relates to GaN MOSFETs and, more particularly, to an enhancement-mode MOSFET with low leakage current and improved reliability.
2. Description of the Related Art
GaN MOSFETS are well known in the art, and are of utilized in high power, high frequency, and high temperature applications. GaN MOSFETS are typically based on the formation of a heterojunction between a GaN region, typically known as the channel layer, and an overlying AlGaN region, typically known as a barrier layer. The GaN channel layer and the AlGaN barrier layer have different band gaps that induce the formation of a two-dimensional electron gas (2DEG) that lies at the junction between the GaN channel layer and the AlGaN barrier layer and extends down into the GaN channel layer.
The 2DEG, which functions as the “channel” of the transistor, produces a high concentration of electrons which causes a conventionally-formed GaN MOSFET to function as a depletion-mode device (nominally on when zero volts are applied to the gate of the device, and the source and drain regions of the device are differently biased).
Although there are applications for depletion-mode GaN MOSFETs, the nominally on state of a depletion-mode transistor requires the use of a control circuit during start up to ensure that source-to-drain conduction within the transistor does not begin prematurely. On the other hand, an enhancement-mode GaN MOSFET (nominally off when zero volts are applied to the gate of the device, and the source and drain regions of the device are differently biased) does not require a control circuit because the transistor is nominally off at start up when zero volts are placed on the gate.
However, to form an enhancement-mode GaN MOSFET, the AlGaN barrier layer must be made thin enough (e.g., a few nm thick) so that when zero volts are applied to the gate of the device, (and the source and drain regions of the device are differently biased) substantially no electrons are present in the 2DEG region, and when a voltage that exceeds a threshold voltage is applied to the gate of the device, (and the source and drain regions of the device are differently biased), electrons accumulate in the 2DEG region and flow from the source region to the drain region.
One problem with reducing the thickness of the AlGaN barrier layer is that high levels of leakage current can pass through the AlGaN barrier layer to the gate, which is conventionally implemented as a Schottky contact. One solution to this problem is to add a gate insulation layer that lies between the AlGaN barrier layer and the gate.
Current-generation, enhancement-mode GaN MOSFETs use a variety of deposited oxides to form the gate insulation layer. These deposited oxides include Al2O3, HfO2, MgO, Gd2O3, Ga2O3, ScO2, and SiO2. Of all of these oxides, SiO2 has a bandgap Eg of 9 eV and a ΔEc to AlGaN that can be as high as 2.5 eV, thereby leading to the lowest leakage current and a threshold voltage as high as 2.5 volts.
One problem with all of these deposited oxides, including SiO2, is that these deposited oxides have a high density of interface states (e.g., greater than 4×1011/cm2) that results in a large number of trapping sites at the junction between the gate insulation layer and the AlGaN barrier layer. Large numbers of trapping sites lead to the breakdown of the gate insulation layer which, in turn, reduces the long-term reliability of the GaN devices. Thus, there is a need for an enhancement-mode GaN MOSFET that has a low leakage current.